Efficient cooling of hot spots made easy

Compact 40 x 40 mm variable-power fan

August 2015

Modern electronic components and assemblies make it possible to achieve a high power density together with a compact design. As a result, more heat does however have to be dissipated from both CPUs and power electronics. Conventional cooling concepts with moderate amounts of air require large heat sinks in such situations and would counteract the desired miniaturisation effect. Active cooling with efficient high-performance fans offers a better alternative for high-performance cooling in data processing systems, inverters and laser devices for example. With an edge length of just 40 mm, such fans convey a powerful flow of air to the hot spot. The high-speed, turbulent flow dissipates heat exactly where it is needed, saving valuable installation space. Intelligent electronic fan control constantly ensures optimum power adjustment and pinpointed variation of the air volume in line with requirements. This reduces both energy consumption and operating noise. High power combined with a compact design offers a variety of advantages: Users benefit from greater effective power, whilst miniaturisation conserves both resources and the environment. This applies not only to computers and control electronics but also to power components such as transistors, thyristors or diodes in frequency converters, welding appliances or inverters. Increased power and miniaturisation are however unfortunately also accompanied by greater losses in each component. New cooling concepts are called for. Which is why the ebm-papst fan specialists from St. Georgen in the Black Forest developed a fan to suit the demands of modern high-performance electronics. The catalogue of requirements included compact dimensions, a very high air conduction rate and pressure increase to be able to provide a sufficient volume of cooling air for even tightly packed components (Figure 1). At the same time the aim was to design a fan which would set new standards in terms of efficiency and variability. This was achieved by developing special control electronics with optimised actuation and a connection for external communication to permit demand-based air volume control.

Figure 2: The complex control board is fully integrated into the fan housing

Compact fan full of expertise

With regard to conveying a certain air flow per unit of time, a small cross-section produces a much higher flow velocity than a larger one. In addition, the infeed of energy is required to move gas molecules – a function performed by the blades of the fan rotors. Similarly to a large propeller on an aircraft, a large impeller conveys a large volume at moderate speed and air velocity. By contrast, a higher blade speed and optimised inlet/outlet geometry are necessary for a small fan to convey the same volume of air per unit of time. With a high speed and high air jet velocity, a small, ideally designed rotor can therefore achieve the air performance level of a large fan rotor. The difficult aspect is the optimum interaction of all components. This was where our development experts in the Black Forest were able to draw on their decades of experience. Two criteria therefore had to be considered for the compact fans with an edge length of 40 x 40 mm and flow rates of nearly 40m³/h: an aerodynamically optimised design for the rotor and housing and a sufficiently powerful drive system. Ideally, a balance should be achieved between the demands for a high speed in terms of aerodynamics and in relation to the electric drive: Smaller electric motors draw their power from a high speed with a relatively moderate torque. In several stages, the powerful external rotor motor of the fan was ideally adapted to the optimised aerodynamic components. This was so successful that the new fan, designed with the standard 40 x 40 x 28 mm dimensions, now attains virtually twice the power of its predecessor.

Air conduction “On Demand”

Intelligent control of the drive system and thus of the air volume must also be possible. For this purpose, the electronically commutated motor is regulated by way of complex control electronics (Figure 2). This permits not only economical, in other words power-saving operation, but also pinpointed air conduction: For instance, the air flow is reduced if a CPU or diode laser is in idle mode, whereas accordingly more cooling air is conveyed if the amount of heat to be dissipated increases under load. In addition to lowering the power requirement and noise level, this also extends the service life of the upstream filters for example. All-in-all, the built-in intelligence of the fan drastically cuts the operating costs of the cooling system. Special functions such as a speed signal, a Go/NoGo alarm, an external temperature sensor, a PWM control input and protection against moisture or even salt spray fog extend the range of possible applications beyond the bounds of switch cabinets or computer centres right through to tough industrial environments.

Figure 3: A small fan with a lot to offer – as the air performance diagram shows!

In practice

The fans in the 420J series are designed and manufactured to ebm-papst “GreenTech” environmental standards. They have extremely compact dimensions of 40 x 40 x 28 mm. Two versions are available with 12 or 24 VDC nominal voltage. Depending on the model the air flow is between 24 and 38 m³/h with a pressure increase of up to 500 pascal and a power consumption of 2.5 or 7,1 W (Figure 3). The fan is quieter than the old version, with a reduction in sound power level of between 2 and 5 dB(A) depending on the operating point. The small fan weighs only 45 g, as the housing and impeller are made of light, vibration-damping GRP (glass fibre-reinforced plastic) such as PBT (polybutylene terephthalate) or PA (polyamide). The fan is suitable for use over a broad temperature range between -20 and +70 °C. At full load, power consumption is around 70% lower than that of its predecessor. The improved motor efficiency level also has a positive effect in terms of bearing heat generation. Years of operation at a nominal speed of up to 17,200 rpm are thus no problem. The service life (L10) values based on ebm-papst’s stringent in-house standard are very high: 75,000 or 32,500 hours for the M-version of the fan at 40°C or 70°C respectively. Employing the usual L10IPC method, the life expectancy of an M-version at 40 °C is around 127,500 hours. The robust plastics can withstand even tough environmental conditions. An extra-robust version of the fans is also available to provide a broader range of applications extending to industrial equipment and mobile units. In this case the electronics board is encapsulated in polyurethane and the stator is provided with a protective coating for example. This enables the fan to pass the salt spray mist test, qualifying it for use in the tough environment of converters or diode welding systems.